1. Field of the Invention
The present invention relates to a printed circuit board having a predetermined wiring pattern corresponding to a desired electric circuit formed by printing, photo-etching, etc. on a board of insulating material, such as a phenolic resin, for supporting thereon various electric components attached by soldering to the printed circuit board via terminal holes provided at predetermined portions of the wiring pattern.
2. Description of the Prior Art
Printed circuit boards are an essential part to make up an electric product. To the printed circuit board, various electric components are attached by soldering. In most cases, the soldering is achieved by using the dip soldering in which all the electric components are soldered at one time to the printed circuit board, with terminals or leads of each electric component stably received in terminal holes in the printed circuit board.
In recent years, downsizing and structural simplification have been a strong desire against products in various fields of art including a device having a printed circuit board.
In the case of the printed circuit board, the downsizing and structural simplification have been achieved by forming a switch assembly using a portion of the wiring pattern forming an electric circuit.
FIGS. 3A through 3C are cross-sectional views showing various forms of conventional switch assemblies each formed by using a portion of a printed circuit board.
The switch assembly shown in FIG. 3A is composed of a copper foil surface 2 constituting a portion of the wiring pattern formed on a printed circuit board 1, a movable contact 3 fixed to the printed circuit board 1 so that an end 3a of the movable contact 3 is selectively engageable with the copper foil surface 2, a slidable actuator 4 movable in the direction of the arrow A for bringing together the end 3a of the movable contact 3 and the copper foil surface 2, and a frame 5 slidably holding thereon the actuator 4.
The switch assembly illustrated in FIG. 3B comprises two copper foil surfaces 6 and 7 constituting a portion of the wiring pattern formed on a printed circuit board 1, a movable contact 8 having two end portions 8a and 8b selectively engageable with the copper foil surfaces 6 and 7, respectively, a pushbutton actuator 9 firmly supporting thereon the movable contact 8 and movable in the direction of arrow B for bringing the end portions 8a, 8b of the movable contact 8 into contact with the copper foil surfaces 6, 7, and a frame 5 movably holding thereon the pushbutton actuator 9.
Similarly, the switch assembly shown in FIG. 3C includes two copper foil surfaces 6 and 7 constituting a portion of the wiring pattern formed on a printed circuit board 1, a movable contact 8 having two end portions 8a and 8b selectively engageable with the copper foil surfaces 6 and 7, respectively, a slidable actuator 10 firmly supporting thereon the movable contact 8 and movable in the direction of arrow C for bringing the end portions 8a, 8b of the movable contact 8 into contact with the copper foil surfaces 6, 7, and a frame 5 slidably holding therein the slidable actuator 10.
Though not shown, each of the conventional switch assemblies also includes a mechanism for holding the actuator 4, 9 or 10 in an operating position in which the movable contact 3 or 8 is in contact with a mating fixed contact consisting of the copper foil surface 2 or surfaces 6 and 7.
In making the switch assembly shown in FIG. 3A, the movable contact 3 must be attached to the printed circuit board 1 after electric components (not shown) are attached by dip soldering to the printed circuit board 1. Accordingly, the production efficiency of this switch assembly is relatively low. In the switch assemblies shown in FIGS. 3B and 3C, the movable contact 8 is attached to the actuator 9 or 10. This construction obviates the need for an additional processing operation subsequent to the dip soldering, such as done in the case of the switch assembly shown in FIG. 3A. The switch assemblies shown in FIGS. 3B and 3C can, therefore, be produced efficiently. Due to this high production efficiency, these switch assemblies are used frequently.
However, the copper foil surface 2, 6 or 7, serving as a fixed contact relative to the movable contact 3 or 8 of the conventional switch assembly, is coated with a solder when the dip soldering is performed to attach the electric components to the printed circuit board 1.
It is known that the amount of solder deposited by dip soldering on predetermined portions of a wiring pattern is not uniform but rather changes largely though it may vary depending on the size of the portions to be soldered. Experiments made by the present applicants have proved that for portions of a printed circuit board to be used as contacts of switch assemblies, the thickness of solder deposited on such portions varies in the range of about 1 mm.
In order to take up this variation of solder thickness, it is necessary to enlarge the stroke of the movable actuator 4, 9 or 10, which enlargement of the stroke, however, will increase the overall size of a device in which the printed circuit board is incorporated.
In addition, even when the stroke of the actuator 4, 9 or 10 is enlarged as described above, a drawback still remains in that if the variation of solder thickness is excessively large, the movable contact 3 or 8, as it is moved by the actuator 4, 9 or 10, may be deformed or otherwise damaged due to abutment with the soldered portions.
In order to reduce the variation of solder thickness, there has recently been proposed an improved process in which the copper foil surface 2, 6 or 7 serving as a fixed contact on the printed circuit board 1 is in the form of a fork in plan view, as shown in FIG. 4, and the dip soldering is performed as advancing in the longitudinal direction of strips of the bifurcated copper foil surface 2, 6 or 7, as indicated by the arrows D and E shown in FIG. 4.
According to the improved process, the variation of solder thickness can be reduced to 0.1 to 0.5 mm. The improved process using the bifurcated copper foil surface 2, 6 or 7 is, however, still unsatisfactory in that because the variation of solder thickness is not reduced to zero, the foregoing problems resulting from the variation of solder thickness still remain unsolved.
As a basic solution for the variation-induced problems, a copper foil surface serving as a contact is covered with a masking material such as a paper tape or a silicone film before the dip soldering is performed. Subsequent to the dip soldering, the masking material is removed.
This solution is effective because the copper foil surface serving as a contact is not subjected to the dip soldering and hence is not coated with solder. Accordingly, the problems caused by the variation in thickness of solder on the copper foil surface do not take place any more.
However, due to the use of the masking material, the foregoing solution needs additional processing operations including application and removal of the masking material and inspection of the presence of the masking material. With this additional processing operations, the production cost of a printed circuit board is increased.